1. Field of the Invention
This invention relates to reducing optical losses in optical fibers having an ultraviolet-cured coating.
2. Description of the Prior Art
A coated optical fiber, as used in various applications including communications, typically includes a tough but flexible polymer coating material which surrounds the optical fiber. The optical fiber itself is typically silica glass, with various additives included to obtain desired optical properties. The coating material is usually designed to protect the optical fiber from scratches and surface abrasion. Additionally, a coating may serve to reduce microbending losses, and allow easier handling of the fiber.
One conventional method of coating an optical fiber is by applying a prepolymer material, typically a liquid, which is then cured on the fiber by heating. More recently, ultraviolet-cured coatings have been developed. These ultraviolet (UV) coating materials typically comprise prepolymer material to which an ultraviolet sensitive photoinitiator material is added; see U.S. Pat. No. 4,099,837, assigned to the same assignee as the present invention. It is also known to produce fibers having dual layer coatings, wherein the inner layer is relatively soft and flexible, in order to further reduce microbending losses. For this purpose, the inner layer typically comprises silicone or comparable compliant buffer materials. The outer layer is typically made of a more abrasion-resistant material, as in the case of single layer coatings. Either or both of these layers could include ultraviolet-cured material.
Although ultraviolet radiation is necessary to cure these coatings, it is known that ultraviolet radiation may increase optical losses in silica glass doped with various materials; see, for example, "The Effect of UV and X-Ray Radiation on Silicate Glasses, Fused Silica, and Quartz Crystals," A. Kats and J. M. Stevels, Philips Research Report 11, pp. 115-156, 1956. It therefore may be necessary in certain circumstances to find ways of preventing the ultraviolet light used to cure the polymer coating from increasing optical losses in optical fibers.